1. Field of the Invention
The present invention relates to lighting devices, and display devices having a lighting device and a light transmission type display panel.
2. Description of the Related Art
An example of the display devices is a liquid crystal display (“LCD”) device. The LCD device includes a liquid crystal panel, and a backlight (i.e., a lighting device) to irradiate the liquid crystal panel with light. The liquid crystal panel displays images on its front by changing the transmittance of the light coming from the backlight. The backlight includes light sources and a housing for confining light emitted from the light sources. Generally, backlights are roughly classified into an edge light type and a direct type according to positions where the light sources are arranged. In LCD devices having a direct type backlight, a plurality of light sources are arranged directly under, i.e., directly on the rear side of, a liquid crystal panel, and a diffusion plate and an optical sheet for diffusing light are arranged between the light sources and the liquid crystal panel. Direct-lit LCD devices are advantageously capable of enhancing contrast of displayed images through local light quantity control of a plurality of light sources. Therefore, LCD devices of this type are adopted widely, in particular for applications in which high image quality is required.
When images are displayed on the liquid crystal display device, heat is generated from the light sources of the backlight. If the generated heat is not sufficiently discharged to the outside of the housing, temperature of the diffusion plate, the optical sheet, and/or the liquid crystal panel increases. In such a case, disadvantageously, the diffusion plate and the optical sheet would be deformed by thermal expansion, and/or liquid crystal molecules filled in the liquid crystal panel would be denatured by the increased temperature, leading to occurrence of display defects.
JP 2009-129707 A discloses an LCD device in which a light source substrate mounted with light sources and a drive substrate for driving the light sources are attached to a bottom chassis made of metal having high thermal conductivity, the bottom chassis being a part of a housing, so as to discharge heat generated from the light source substrate and the drive substrate via the bottom chassis to the outside of the housing.
When LCD devices are used for digital signages (electronic signboards), for example, they may be required to have improved luminance, or brightness, in order to enhance the visibility or legibility of images which are displayed outdoor, or in a building illuminated by high-brightness illumination. To increase the luminance of the LCD devices, it is necessary to increase a current to be inputted to the light sources included in the backlight to thereby increase the emission intensity of the light sources.
However, increasing of the current to be inputted to the light sources to increase the emission intensity of the light sources causes simultaneous increase of an amount of heat generated from the light sources. In such a case, with the LCD device as disclosed in JP 2009-129707 A, heat generated from the light sources would not be sufficiently discharged to the outside of the casing, so that the temperature of the diffusion plate, the optical sheet, and/or the liquid crystal panel would increase and display defects would occur. Thus, in the liquid crystal display device of JP 2009-129707 A, it is difficult to increase the luminance without causing display defects.
There are, therefore, a need for lighting devices which can efficiently discharge heat generated from light sources to the outside of a housing, and a need for display devices which can increase luminance without causing display defects.